The present invention relates to lithium cells based on lithiated transition metal titanates. More particularly, it provides active materials useful as a cathode (positive electrode) active material and an anode (negative electrode) active material for use in secondary electrochemical cells.
Lithium batteries are prepared from one or more electrochemical cells. Non-aqueous lithium electrochemical cells typically include a negative electrode, a lithium electrolyte prepared from a lithium salt dissolved in one or more organic solvents, and a positive electrode of an electrochemically active material, typically a chalcogenide of a transition metal. During discharge, lithium ions from the negative electrode pass through the liquid electrolyte to the electrochemically active material of the positive electrode, where the ions are taken up with the simultaneous release of electrical energy. Thus on discharge, the positive electrode functions as a cathode, and the negative electrode as an anode. To reflect this fact, the terms xe2x80x9cpositive electrodexe2x80x9d and xe2x80x9ccathodexe2x80x9d will be used interchangeably in the description and claims, as will the terms xe2x80x9cnegative electrodexe2x80x9d and xe2x80x9canodexe2x80x9d. During charging, the flow of ions is reversed so that lithium ions pass from the positive electrode through the electrolyte and are plated back onto the negative electrode.
Recently, the lithium metal anode has been replaced with a carbon anode such as coke or graphite in which lithium ions can be inserted to form LixC6. In the operation of the cell, lithium passes from the carbon through the electrolyte to the cathode where it is taken up just as in a cell with a metallic lithium anode. During recharge, the lithium is transferred back to the anode where it re-inserts into the carbon. Because no metallic lithium is present in the cell, melting of the anode does not occur even under abusive conditions. Also, because lithium is reincorporated into the anode by insertion or intercalation rather than by plating, dendritic and spongy lithium growth does not occur. Non-aqueous lithium electrochemical cells are discussed, for example, in U.S. Pat. Nos. 4,472,487, 4,668,595, and 5,028,500. These cells are often referred to as xe2x80x9crocking chairxe2x80x9d batteries because lithium ions move back and forth between the insertion or intercalation compounds during charge/discharge cycles.
Known positive electrode active materials include LiCoO2, LiMn2O4, and LiNiO2. The cobalt compounds are relatively expensive and the nickel compounds are difficult to synthesize. A relatively economical positive electrode is LiMn2O4, for which methods of synthesis are known. The lithium cobalt oxide, the lithium manganese oxide, and the lithium nickel oxide have a common disadvantage in that the charge capacity of a cell comprising such cathodes may suffer a significant loss in capacity. That is, the initial capacity available (amp hours/gram) from LiMn2O4, LiNiO2, and LiCoO2 is less than the theoretical capacity because significantly less than 1 atomic unit of lithium engages in the electrochemical reaction. Such an initial capacity value is significantly diminished during the first cycle operation and such capacity further diminish in successive cycles of operation. For LiNiO2 and LiCoO2 only about 0.5 atomic units of lithium is reversibly cycled during cell operation. Many attempts have been made to reduce capacity fading, for example, as described in U.S. Pat. No. 4,828,834 by Nagaura et al. However, the presently known and commonly used, alkali transition metal oxide compounds suffer from relatively low capacity. Therefore, there remains the difficulty of obtaining a lithium-containing electrode material having acceptable capacity without disadvantage of significant capacity loss when used in a cell.
Japanese Patent No. 08180875 to Aichi Seiko discloses a lithium secondary battery having a cathode made of an active material consisting of a lithium metal titanate of structure LiTiMO4 wherein M is manganese, iron, chromium, nickel, cobalt, magnesium, and/or boron.
Lithium ion technology, and the associated lithium containing compounds useful as cathode active materials in such batteries, have given the industry needed flexibility in designing electrochemical cells for a wide variety of technological uses. However, the industry is constantly seeking for new materials to provide even greater flexibility in design parameters, ease of construction, and increased energy density.